使用simpledateformat，规定好格式，parser出错即为非法/** * calculates the number of days between two calendar days in a manner * which is independent of the calendar type used. * * @param d1    the first date. * @param d2    the second date. * * @return      the number of days between the two dates.  zero is *              returned if the dates are the same, one if the dates are *              adjacent, etc.  the order of the dates *              does not matter, the value returned is always >= 0. *              if calendar types of d1 and d2 *              are different, the result may not be accurate. */static int getdaysbetween (java.util.calendar d1, java.util.calendar d2) {    if (d1.after(d2)) {  // swap dates so that d1 is start and d2 is end        java.util.calendar swap = d1;        d1 = d2;        d2 = swap;    }    int days = d2.get(java.util.calendar.day_of_year) -               d1.get(java.util.calendar.day_of_year);    int y2 = d2.get(java.util.calendar.year);    if (d1.get(java.util.calendar.year) != y2) {        d1 = (java.util.calendar) d1.clone();        do {            days += d1.getactualmaximum(java.util.calendar.day_of_year);            d1.add(java.util.calendar.year, 1);        } while (d1.get(java.util.calendar.year) != y2);    }    return days;}